Automotive vehicles such as, for example, motorcycles and four wheeled vehicles make use of a catalytic converter disposed on an exhaust passage for removing HC (hydrocarbon), CO (carbon monoxide) and NOX (nitrogen oxides) components, contained in exhaust gases emitted from a combustion engine, by means of a catalytic reaction. This is disclosed in, for example, the JP Laid-open Patent Publication No. S60-17220, first published Jan. 29, 1985. The catalytic converter referred to above is of a structure including a honeycomb carrier structure carrying platinum and rhodium deposited on a multiplicity of surfaces thereof.
In an attempt to reduce the size and the cost of manufacture of the catalytic converter, it has hitherto been contemplated to increase the number of cells, which are elongated pores in the honeycomb carrier structure, to thereby increase the cell density and also to reduce the amount of rhodium, which is an element employed as a major catalyst, that is used in the catalytic converter. More specifically, increase of the cell density results in increase of the total surface area of the honeycomb carrier structure (hereinafter referred to as the honeycomb surface area, which in turn results in increase of the capability of substantially purifying the exhaust gases, and, therefore, for a given exhaust gas purifying performance, the honeycomb carrier structure can be reduced in size and manufactured at a reduced cost. On the other hand, although rhodium is a catalyst element currently considered the most excellent catalyst in exhaust gas purifying capability, it is expensive, and therefore, the manufacturing cost would be reduced considerably if the intended exhaust gas purifying capability can be made available while the amount of rhodium used is reduced.
It has, however, been found that if the cell density is increased to a value equal to or higher than about 400 cells per square inch, the maximum catalyst internal temperature, which is induced when unburned components of the exhaust gases flow from the combustion engine, will increase with increase of the cell density, but the minimum catalyst internal temperature on the other hand will scarcely change, resulting in increase of the difference in temperature between the maximum and minimum temperatures. Also, in order to increase the cell density, it is needed to increase the thickness of each of annular flat plates and annular undulated plates both forming respective parts of the honeycomb carrier structure. Accordingly, the durability of the flat and undulated plates, each having a small thickness, will be lowered when they are repeatedly thermally contracted by the effect of a relatively large difference in temperature between those catalyst internal temperatures.
It is to be noted that the catalyst internal temperature referred to hereinbefore and hereinafter means the temperature occurring internally of a tubular casing of the catalyst converter, in which a multiplicity of cells are arranged, not the temperature of an outer peripheral surface of the tubular casing of the catalyst converter.
On the other hand, since rhodium is a major catalyst element excellent in overall capability of substantially removing particularly both of HC and CO compounds as compared with that of any other catalyst elements, the amount of rhodium carried is hardly reduced in the face of the required or desired exhaust gas purifying performance.